The present invention relates to a combustion method and apparatus.
A combustion apparatus such as waste incinerator is generally used in such a manner that domestic waste and other refuse are charged into a combustion chamber and are burnt. In the chamber, the waste and refuse are thermally decomposed and unreacted gas generated by the thermal decomposition is burnt. Through these two processes, waste and refuse are burnt.
In a conventional combustion apparatus, these two types of reactions, i.e. thermal decomposition of the waste and combustion of unreacted gas, are carried out simultaneously in one and the same combustion chamber, which tends to cause insufficient execution of both thermal decomposition of the waste and combustion of the unreacted gas.
Thus, in the thermal decomposition of waste and refuse, temperature change due to influence of the combustion of unreacted gas tends to make the rate of thermal decomposition unstable.
In the combustion of unreacted gas, air may be insufficiently mixed with the gas in the combustion chamber, resulting in incomplete combustion of unreacted gas and generation and discharge of harmful substances to the atmosphere.
For this reason, a combustion apparatus has been proposed in recent years in which the existing combustion chamber is used as a primary combustion chamber and a secondary combustion chamber is added. The primary combustion chamber is exclusively used for thermal decomposition of the waste whereas unreacted gas generated by thermal decomposition is guided to the secondary combustion chamber for complete combustion of the unreacted gas.
FIG. 1 represents a currently proposed combustion apparatus having a secondary combustion chamber.
In the figure, reference numeral 1 denotes a primary combustion chamber such as waste incinerator having a waste inlet 2 and a hopper 3 at its upper and lower portions, respectively; 4, a plurality of horizontal air distribution pipes mutually spaced and arranged above the hopper 3; 5, a fluidized bed formed by a fluidization medium such as fluidization sand fluidized by air from the pipes 4; and 6, a burner mounted on an intermediate portion of the chamber 1 and directed to the fluidized bed 5.
Reference numeral 7 denotes a medium outlet on a lower end of the hopper 3; 8, a medium inlet on the intermediate portion of the chamber 1; 9, a medium circulating passage such as a bucket elevator for connection of the outlet 7 with the inlet 8; 10, a medium conveyor at a entrance side of the passage 9; and 11, a sieve at exit side of the conveyor 10.
Reference numeral 12 represents a generally cylindrical secondary combustion chamber above the primary combustion chamber 1 and having a gas outlet 13 and a hopper 14 on its upper and lower portions, respectively; 15, unreacted gas generated in the chamber 1 and having unburnt solids 16 entrained therein; and 17, an ash outlet on a lower end of the hopper 14.
Reference numeral 18 denotes a throttled passage through which the unreacted gas 15 rises up from the chamber 1 to the chamber 12; and 19, a connection tangentially connected to a lower portion of a side wall of the chamber 12. Reference numeral 20 represents an air passage connected to a ventilator 21 located externally; 22, a primary air passage branched off from the passage 20 for supplying air to the pipes 4; 23 and 24, secondary air passages branched off from the passage 20 for supplying air to points on the passage 18; and 25, 26 and 27, valves in the passages 22, 23 and 24, respectively.
By operating the ventilator 21, air is supplied to the pipes 4 through the passages 20 and 22 to fluidize the bed 5 in the chamber 1 while the bed 5 is preheated by the burner 6. Under this condition, waste is charged through the inlet 2 into the chamber 1.
The waste charged into the chamber 1 is then thermally decomposed in the preheated fluidized bed 5 and the unreacted gas 15 and the unburnt solids 16 such as char are generated by the thermal decomposition.
With the air being fed through the pipes 4 to the chamber 1, the unburnt solids 16 are burnt and the resultant combustion heat promotes thermal decomposition of the waste.
The unreacted gas 15 generated by thermal decomposition of the waste rises up to the secondary combustion chamber 12 via the passage 18 and the connection 19. As a result, in the chamber 1, the waste can be thermally decomposed at a given rate without being influenced by combustion of the unreacted gas 15.
On the other hand, the unreacted gas 15 and part of the unburnt solids 16 directed through the passage 18 and the connection 19 toward the chamber 12 are fed, at the passage 18, with the air from the passages 23 and 24 and are mixed with the same to some extent and then are tangentially introduced into the generally cylindrical secondary combustion chamber 12.
In the chamber 12, a spiral ascending flow is formed by the unreacted gas 15 mixed with the air and mixing of the unreacted gas 15 with the air is further promoted by the spiral ascending flow. In addition, retention time necessary for combustion of the unreacted gas 15 in the chamber 12 is sufficiently assured by the spiral flow. During this retention time, complete combustion of the unreacted gas 15 occurs, which will reduce generation of harmful substances due to incomplete combustion.
Combustion gas generated by the combustion is discharged through the outlet 13 on the upper end of the chamber 12.
Apart from the above, in the chamber 1, part of the medium constituting the fluidized bed 5 is sent through the outlet 7 and the conveyor 10 to the sieve 11 where unburnt materials are removed from the medium. Then, the medium is circulated via the passage 9 and the inlet 8 to the chamber 1.
Further, the unburnt solids 16 rising up through the passage 18 and entrained in the unreacted gas 15 are centrifuged by the spiral ascending flow in the chamber 12 and are discharged as ashes through the outlet 17 on the lower end of the hopper 14.
The secondary combustion chamber in the above-mentioned combustion apparatus has the following problem.
Since thermal decomposition of the waste and combustion of the unreacted gas 15 are exclusively carried out in the first and secondary combustion chambers 1 and 12, respectively, the waste can be burnt under the condition closer to complete combustion. However, the combustion gas discharged through the outlet 13 at the upper end of the chamber 12 still contains carbon monoxide at an concentration of about 10 ppm. This is because the unburnt solids 16 entrained in the unreacted gas 15 are not completely burnt out in the chamber 12. There is still room for further improvement on the combustion condition in the secondary combustion chamber 12.
It is therefore an object of the present invention to provide a combustion apparatus which can carry out substantially complete combustion.